1. Field of the Invention
Network nodes and other equipments in large-capacity, highly functional optical communication systems are demanding various types of processing performed on ultrahigh-speed optical data signals be implemented directly in an optical domain without converting them to electrical signals. The present invention relates to all-optical signal processing technologies that are desired to be performed on ultrahigh-speed optical data signals, such as switching, regeneration and modulation format conversion.
2. Description of the Related Art
Optical communication systems are developing from technologies pertaining to simple transmission between two points, to network technologies encompassing multiple nodes. Nodes placed over a network provide various functions, including regeneration of signals deteriorated as a result of transmission, switching such as route setting, gateway to connect to other networks of different speeds and protocols, and monitoring of signal quality. Today, these series of signal processing is performed using electronic circuits. In this process, optical signals must be converted to electrical signals, and the converted electrical signals must be converted back to optical signals again. At nodes where optical signals are processed at bit rates of 10 to 160 Gb/s, however, it is difficult to implement the signal processing using traditional electronic circuits, due partly to the limited operating speeds of electronic circuits, and partly to the increased power consumptions associated with optical-to-electrical and electrical-to-optical signal conversions. For this reason, technologies that allow optical signals to be processed directly at high speed, without converting them to electrical signals, are required. To address such demand, an optical signal processing circuit having such functions as optical gate switching for switching optical signals at high speed, optical signal regeneration, and modulation format conversion, is needed. Also required is an optical signal processing circuit capable of modulating and controlling the phase of signal light using control light, because the optical signal modulation formats include not only those based on simple on/off keying of light, but also the differential phase shift keying modulation format and other formats whereby the phase of optical waves is modulated.
Pertaining to optical gate switch circuits for controlling and processing optical signals with bit rates of 10 to 160 Gb/s using another optical signals, methods combining the cross phase modulation effect of a semiconductor optical amplifier with an optical interferometer are reported, along with methods that utilize the Kerr effect in optical fibers.
For example, Patent Literature 1 proposes an optical gate switch element whose structure comprises semiconductor optical waveguides exhibiting nonlinear refractive index change against the intensity of control light, being placed symmetrically in two arms of a Mach-Zehnder interferometer. The relaxation time (switch off time) of nonlinear refractive index change occurring in a semiconductor optical waveguide is limited to several nanoseconds due to the relaxation time of the carrier. Accordingly, the element described in Patent Literature 1 utilizes optical interference to cancel the slow relaxation component and thereby achieve gate switch operation at high speed on the order of picosecond. In this configuration, however, it is difficult to match the characteristics of the semiconductor optical waveguides placed in two arms, which presents a number of problems such as a need for a circuit that compensates the differences between the two sets of characteristics.
To solve the above problems, Patent Literature 2 proposes an optical gate switch element offering excellent long-term stability, capable of achieving the same functions provided by the circuit in Patent Literature 1 with only one semiconductor optical waveguide.
However, optical phase change arising from refractive index change caused by control light is accompanied by relaxation of several nanoseconds, and therefore it is still difficult to achieve a phase modulation/control element operating at high speed, even when the optical gate switch element is realized with only one semiconductor optical waveguide.
Patent Literature 3 proposes an optical gate switch circuit of nonlinear optical loop minor type that utilizes the Kerr effect in optical fibers. The nonlinear refractive index change occurring in optical fibers is substantially smaller than the nonlinear refractive index change occurring in semiconductor optical waveguides like those used in Patent Literatures 1 and 2. For this reason, it is necessary to use an optical fiber of several hundred meters to several kilometers in length to extend the length of interaction between control light and signal light, and the power of control light needed to implement switching also becomes higher by more than an order of magnitude. A loop is constituted by an optical fiber, and the interference between lights propagating clockwise and counterclockwise along the loop is utilized to convert to intensity change the optical phase change arising from refractive index change. The Kerr effect of an optical fiber represents a nonlinear refractive index change occurring in the optical fiber due to control light, and exhibits a high-speed response of around several hundred femtoseconds. Accordingly, it is possible to achieve switching operation at several 100 Gb/s or higher, and therefore operation of an optical phase modulation/control element at high speed is theoretically feasible. However, the need for a long optical fiber makes it difficult to adjust signal timings, and there is also a possibility of signal delay. In this sense, this method is not suitable for applications requiring size reduction and/or integration.    [Patent Literature 1] Japanese Patent Laid-open No. Hei 7-20510    [Patent Literature 2] Japanese Patent Laid-open No. Hei 8-179385    [Patent Literature 3] Japanese Patent Laid-open No. 2006-30295